This invention pertains to sensing current through a bonding wire, or similar type of conductor, in close proximity to Hall effect devices.
It is often desired to measure current through an electrical circuit. For example, FIG. 1 shows a typical prior art load circuit wherein a switching device, in this example an MOS field effect transistor (MOSFET) 10, is controlled by an input signal applied to terminal 14 for selectively applying a voltage source VCC available on drain 13 of MOSFET 10 to output terminal 12 connected to source 15 of MOSFET 10. In this manner, an external terminal 12 is available for connection to load device 11, which has its other terminal connected to ground. Thus, in response to the input signal applied to control terminal 14 connected to the gate of MOSFET 10, power supply VCC is selectively connected to load device 11. In this example, it is often times helpful to have an indication of the current through load device 11, which is that current through lead 15.
FIG. 2 shows a prior art circuit similar to that of FIG. 1 in which sense resistor 25 is connected in series between source 15 of MOSFET 20 and load terminal 22. In the example of FIG. 2, current through load device 21 can be measured by measuring the voltage drop VS across current sensing resistor 25. Preferably, the resistance RS of sense resistor 25 is made relatively small compared to the resistance RL of load device 21 in order to minimize added voltage drop across sense resistor 25. However, in practice this is difficult and the resistance of current sense resistor 25 may effectively double the voltage drop between supply voltage VCC and load terminal 22, as compared with the voltage drop between those points in a circuit which uses MOSFET 20 without the inclusion of current sense resistor 25. Any additional drop in power supply voltage to load device 21 is undesirable.
Another prior art technique for sensing current in this fashion is described by Wrathall U.S. Pat. No. 4,553,084 which is hereby incorporated by reference. Wrathall in effect provides a switching circuit between a positive supply voltage and a load terminal which includes a first MOSFET, and a second MOSFET connected in series with a current sense resistor. The first MOSFET is made to be rather large and thus has a high current carrying capacity. The second MOSFET is made to be rather small, and thus carries significantly less of the load current. This allows measurement of the load current without adding additional voltage drop due to the inclusion of a current sense resistor.
However, this approach has several drawbacks. For example, for accurate sensing, the device of Wrathall requires a nulling amplifier as a current sensing mechanism, instead of a resistor, because the ratio of currents in the two sources will not be the same as the ratio of the number of cells in the load portion of the device to the number of cells in the sense portion of the device unless the two sources are at the same voltage. Secondly, when the structure of Wrathall is used in a common source configuration (i.e. when the load is connected to the drains, and the source of the load portion of the device is connected to ground), two power supplies are needed in order to establish a current sink for the sense portion of the device and force the two sources to be at the same voltage. In the common drain configuration (i.e. when the drains are connected together to a power supply and the sources are used to source current to the load device) the nulling amplifier must withstand the full common-mode voltage excursion on the load device and, if it is desired to detect a short-circuit load, the nulling amplifier must have a common mode range equal to the power supply voltage. Lastly, the current sense only works when the device is "ON". This limits its usefulness in driving inductive loads which can produce reverse polarity current in the body-drain diode of the device. Sensing the presence and magnitude of this reverse polarity current provides useful information for system control as well as the indication of fault conditions.
These restraints are rather limiting making design of a circuit as described by Wrathall difficult, bulky, and expensive.